The objective of this project is to elucidate mechanisms of attachment of human JCV. Attachment mechanisms of related polyomaviruses are known in atomic detail, and involve interactions between the viral capsid protein VPl and cell surface receptors that are ganghosides. Little is known about the molecular basis of JCV attachment. We have overexpressed the VPl capsid protein of JCV and obtained small crystals. An integrated research program is proposed to (i) determine high-resolution structures of the JCV capsid protein VPl in complex with oHgosaccharide receptors (ii) determine structures of the JCV capsid protein VPl in complex with relevant portions of the serotonin receptor 5HT2aR, and (iii) determine high-resolution structures of the JCV capsid protein VPl in complex with inhibitors. These aims should advance our understanding of JCV attachment to cells and point out strategies to intervene with the receptor interactions. As very few structure-function studies of virus-receptor interactions have been performed, the broader impact of this work will be in revealing general mechanisms by which pathogenic viruses recognize cellular receptors and cause organ-specific disease. We envision a high level of synergy between this project and projects 2 and 3, for the following reasons. The studies proposed here will provide a solid structural basis for understanding the binding modes of receptors and inhibitors to JCV, and this knowledge can then be directly used by the other projects to establish functional assays and to facilitate the development of effective small-molecule inhibitors. Vice versa, discoveries made by the other groups such as the identification of alpha-defensin as a JCV inhibtor can be directly used by us to provide a structural explanation for this interaction, which in turn can be used as platform for the development for other, strongly inhibitory ligands.